1. Field of the Invention
The present invention relates to a process for treating an impure thermoplastic polymer, such as raw polymers and recycled polymers, during passage thereof through an extrusion apparatus to separate and remove low molecular weight compounds therefrom. More particularly, the process according to the present invention relates to a novel procedure for extracting at least a part of the low molecular weight compounds from the plasticized impure thermoplastic polymer into a separation agent and removal of the separating agent charged with low molecular weight compounds from the extrusion apparatus prior to thermal vacuum treating the plasticized polymer to vacuum degas the plasticized polymer and thereby further remove separating agent and low molecular weight compounds from the plasticized polymer. The combination of solvent extraction in the extrusion apparatus with subsequent vacuum degassing results in removal of large amounts of low molecular weight compounds with a relatively small expenditure of energy so that energy-intensive vacuum degassing is needed only for removal of residual separation agent and low molecular weight compounds.
2. Description of the Related Art
A process of this type is known from Federal Republic of Germany Published Application No. 39 32 793 A1, which corresponds to U. S. Pat. No. 4,963,650, the disclosure of which is incorporated herein by reference, for the treatment of a polyamide-6 melt with steam. This document points out that, when saturated steam is used, it is also possible that the steam might escape from the extruder in a direction counter to the movement of the melt if the pressure distribution is improperly set and should be avoided per se. However, the invention dealt with in Federal Republic of Germany Published Application No. 39 32 793 A1 relates to a repeated treatment of polycaproamide melts and their copolymers in an extruder with superheated steam and thermal vacuum degassing for separating low molecular compounds therefrom.
Even though the amount of water to be introduced into the polymer melt ahead of each degassing zone in this published Application is limited to 150%, referring to the low molecular weight compounds initially contained in the original melt, the amount of water to be charged and removed makes considerable demands on the capacity of the vacuum installation. This installation is required to continuously maintain a vacuum in all low pressure zones for separating the charged water in order to obtain a residual amount of low molecular weight components of at most 1.8% at the end.
Also, in contrast to the expert opinion expressed in Federal Republic of Germany Published Application No. 39 32 793 A1 not to permit a counterflow in the course of purifying polyamide-6 with saturated steam, U.S. Pat. No. 3,799,234, the disclosure of which is incorporated herein by reference, expressly proposes a counterflow of separating agent in such a process, which separating agent may be steam in certain cases for the separating treatment of plastics. A vacuum treatment is also provided as an alternative to aiding the removal of the charged separating agent. However, the requirement in U.S. Pat. No. 3,799,234 that the gas must be inert and that a suitable one must be selected from the multitude of inert gases does not make it clear whether a thermal vacuum treatment of polymers is also intended, similar to that of polymer condensates.
A thermal treatment for separating low molecular weight compounds from thermoplastic polymers in an extruder with an underpressure as the residual pressure or, alternatively, with exhaust into the atmosphere as the upper pressure limit, can be found in the two references mentioned above. This exhaust into the atmosphere is the lower limit in a non-species-related, same direction flow process known from U.S. Pat. No. 3,683,511, the disclosure of which is incorporated herein by reference, wherein residual pressures of up to 10 bar are provided in order to drive highly volatile hydrocarbons from the elastomers.
Finally, other processes for the thermal vacuum treatment of polymer melts in an extruder are treated in Federal Republic of Germany Published Patent Application No. 39 32 793 A1, such as the already mentioned introduction of saturated steam.
The various processes have been developed with the aim of purifying polymer melts by removing undesired components and to achieve the required quality in this way. Degassing in vacuum with or without the use of entrainers has gained acceptance in practical applications as the most important technique, however, the energy requirements of such prior art processes are quite high.
It is therefore an object of the present invention to further develop the known continuous process and to provide an operationally dependable and efficient process for separating low molecular weight compounds from impure thermoplastic polymers, particularly for lactam removal from polyamide-6, which results in low residual amounts of low molecular weight compounds, i.e., in extensive purification of the polymer, with a relatively low energy expenditure.
It is a further object of the present invention to create a flexible process wherein a thermoplastic polymer in solid form, such as a granulate, chip, powder, etc., can be used as the initial material to be treated and introduced in solid form or in a molten state, for example, from a polyaddition reactor, or in the form of a polymer solution or polymer paste, and which can be advantageously combined with further processing steps, such as admixing additives, granulation or subsequent further processing, for example, into spun fibers.